Phenotypic and functional studies of the PD-1 molecule on CD4+ and CD8+ T cells performed on PBMC samples from uninfected and SIV infected rhesus macaques (RM) revealed 3 major findings: A) a rapid upregulation of PD-1 expression on tetramer positive CD8+ T cells from MamuA.01+ SIV infected macaques early post infection while upregulation of PD-1 on total CD8+ T cells was undetectable. B) In contrast, CD4+ T cells PD-1 expression was markedly higher in total CD4+ T cells during chronic infection and c) there was a correlation between the level of PD-1 expression on naive and central memory CD4+ T cells and the levels of viral loads. Such association was supported further by the finding of a marked decrease of PD-1 expression noted on tetramer positive CD8 T cells as well as on CD4+ T cells shortly after initiation of antiretroviral therapy and downregulation of viral replication in vivo. We have cloned rMamu PD-1 and fused it to a modified Rhesus IgG2 Fc molecule that is unable to bind complement or FcR on NK cells. SIV specific proliferation of CD8 but predominantly CD4 T cells from chronically infected monkeys was markedly amplified with the addition of such soluble PD-1 antagonist in a dose dependent manner. We therefore propose to use such a reagent to address the effect of PD-1-ligand blockade first in vitro on CD4 effectors and antigen specific CD8+ T cells as well as the relative role of PD-1 ligation on Treg cells and the potential DC subset most likely responsible for such signaling. In a second aim, blockade of the PD1-ligand interaction will be attempted during early and late chronic SIV infection in vivo to investigate whether such therapeutic attempts have the capability of restoring/enhancing anti-viral CD4 and CD8+ T cell effectors responses without inducing autoimmunity and prevent SIV mediated disease progression. PUBLIC HEALTH RELEVANCE: The immune system benefits from several partially overlapping negative feedback control mechanisms by which continuous immune activation and inflammation are limited even if the immune insult such as a tumor, transplanted organ or chronic infection persist over time, although during chronic infection such mechanisms also limit the immune responses against infection. Thus, interfering with such mechanisms such as the one termed PD-1 has the potential to enhance antiviral responses in chronic HIV infection. In the current proposal, we aim to investigate the role of PD-1 blockade in the rhesus macaque simian immunodeficiency virus (SIV) model of human AIDS as a therapeutic strategy during chronic infection as well as the safety of such immunotherapy.